Recently, in a rapidly-developing mobile communication system (for example, Personal Handyphone System: hereinafter, referred to as “PHS”), a method of extracting a reception signal from a desired, specific mobile terminal with an adaptive array processing in a radio base station system in communication between a radio base station system and a mobile terminal has been proposed.
In the adaptive array processing, based on a reception signal from a mobile terminal, a weight vector consisting of reception coefficients (weights) for respective antennas in the radio base station system is calculated to perform adaptive control. Thus, a signal from a specific mobile terminal is accurately extracted.
In the radio base station system, a reception weight vector calculator calculating such a weight vector for each symbol of the reception signal is provided. The reception weight vector calculator performs a processing to converge the weight vector so as to reduce a mean square error between the sum of complex multiplication of the reception signal by a calculated weight vector and a known reference signal, that is, the adaptive array processing to converge reception directivity from a specific mobile terminal.
In the adaptive array processing, such a weight vector is adaptively converged in accordance with time and fluctuation of a property of a propagation path for a signal radio wave. In addition, an interfering component or noise is eliminated from the reception signal, to extract the reception signal from the specific mobile terminal.
FIG. 6 is a functional block diagram functionally illustrating a conventional adaptive array processing performed with software by a digital signal processor (DSP) of the radio base station system.
Referring to FIG. 6, reception signals forming a reception signal vector from a mobile terminal, respectively received at a plurality of antennas in the radio base station system, for example, n antennas A1, A2, . . . , An, are switched to a reception circuit side by switching circuits S1, S2, . . . , Sn, and converted to digital signals by not-shown A/D converters respectively.
These digital signals are provided to a not-shown DSP in the radio base station system, and are subjected to adaptive array processing with software later, as in the functional block diagram shown in FIG. 6.
Referring to FIG. 6, the reception signals switched to the reception circuit side by switching circuits S1, S2, . . . , Sn are provided to one inputs of multipliers MR1, MR2, . . . , MRn respectively as well as to a reception weight vector calculator 12.
Reception weight vector calculator 12 calculates a weight vector consisting of weights for respective antennas with adaptive array algorithm described later, and provides the weights to the other inputs of multipliers MR1, MR2, . . . , MRn respectively. Then, the weights are subjected to complex multiplication by the reception signals from corresponding antennas. An adder 10 will provide an array output signal, which is the sum of complex multiplication results.
The result of the sum of complex multiplication as described above is once demodulated to bit data by a demodulation circuit 11. Thereafter, the result is supplied as the array output signal, and remodulated by a remodulation circuit 14.
A known reference signal d(t) stored in a memory 15 in advance is provided to reception weight vector calculator 12 by a switching circuit 13 during a reference signal period defined by a count value of a counter 16, and used for calculation of the weight vector with the adaptive array algorithm. The reference signal d(t) is the known signal common to all users; contained in the reception signal from the mobile terminal. For example, in the PHS, sections in the reception signal, that is, a preamble (PR) and a unique word (UW) constituted with a known bit train, are used.
On the other hand, when an end of the reference signal period is detected with the count value of counter 16, the array output signal remodulated by remodulation circuit 14 is provided to reception weight vector calculator 12 by switching circuit 13, and used for calculation of the weight vector with the adaptive array algorithm.
Reception weight vector calculator 12 uses the adaptive array algorithm such as RLS (Recursive Least Squares) algorithm or SMI (Sample Matrix Inversion) algorithm.
Such RLS algorithm and SMI algorithm are well-known techniques in the field of adaptive array processing, and described in detail, for example, in Nobuyoshi Kikuma, “Adaptive Signal Processing with Array Antenna”, Science Press, Inc., p.35-p.49, chapter 3, “MMSE Adaptive Array”. Therefore, description thereof will not be provided.
In addition, a transmission signal from a not-shown transmission signal source is modulated by a modulation circuit 17, and provided to one input terminals of respective multipliers MT1, MT2, . . . , MTn. The weights calculated in reception weight vector calculator 12 is copied and applied to the other input terminals of respective multipliers MT1, MT2, . . . , MTn.
As described above, the transmission signals weighted by complex multiplication by the weight vector are selected in switching circuits S1, S2, . . . , Sn, and transmitted through antennas A1, A2 . . . , An.
Signals transmitted through the same antennas A1, A2 . . . , An as in reception are weighted by the weight vector targeted for the specific mobile terminal as with the reception signal. Therefore, radio wave signals transmitted from these antennas are emitted with transmission directivity targeted for the specific mobile terminal.
Meanwhile, in the conventional radio base station system in FIG. 6, signals are communicated using n antennas. As the number of antennas increases, interference that can be suppressed through the adaptive array processing increases. Furthermore, antenna gain becomes larger, and coverage of a transmission radio wave is expanded. In other words, as the number of antennas increases, performance of the adaptive array processing will improve.
With regard to the adaptive array algorithm such as RLS and SMI as described above, however, in order to converge the weight vector, that is, to converge reception directivity from the specific mobile terminal, a reference signal of a signal length (the number of symbols) not smaller than twice the number of antennas is required. In other words, when the signal length of the reference signal is predetermined, and if there are antennas more than half the signal length, the weight vector does not converge, nor does the reception directivity.
For example, in the PHS standard, the reference signal consisting of the aforementioned preamble and unique word includes 12 symbols. In other words, the signal length is 12. Therefore, the weight vector can converge, if up to 6 antennas, which is half of signal length 12, at the maximum are used. When more than 6 antennas are used, however, the weight vector can no longer sufficiently converge.
The reason why the number of antennas that allows convergence of reception directivity is restricted by the signal length of the reference signal is well-known in the field of adaptive array processing, as described in detail, for example, in Hiroshi Suzuki et al., “Dynamic Performance Analysis on RLS Adaptive Equalizers for Mobile Radio Transmission”, IEICE Trans, B-II, Vol. J76-B-II, No.4, p.189-p.201, April 1993. Therefore, description thereof will not be provided.
As described above, in order to basically enhance the performance of the adaptive array processing (to achieve wider coverage), the number of antennas should be increased. For that purpose, the signal length of the reference signal should be made longer due to the restriction of the adaptive array algorithm described above. On the other hand, in the PHS standard, for example, an amount of information for 1 frame of a transmission signal consisting of the reference signal and a data signal is determined as 120 bits. If the reference signal length is extended, an amount of user data that can be transmitted will decrease, and data throughput will be lowered.
Therefore, an object of the present invention is to provide a radio base station system which can perform adaptive array processing with a larger number of antennas, using a reference signal of a limited signal length.
Another object of the present invention is to provide a method of controlling directivity, with which reception directivity in a radio base station system can be converged, using a reference signal of a limited signal length.
Yet another object of the present invention is to provide a radio base station system as well as a method of controlling directivity, which can increase antenna gain to expand a coverage of a transmission radio wave, using a reference signal of a limited signal length.